Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/215

 always some which allow the passage of albumen. Or they may become useless subsequently, generally becoming permeable to albumen. This, however, only occurs when they are handled improperly. They must not be cleaned with a rough brush, nor must they be boiled for too long a time. Tubes may become impermeable to peptones through over-boiling, so that, though they should be thoroughly washed, they should be boiled but slightly. They must be kept in sterilized water with a thick layer of toluol (see p. 162), and must never be left for a long time unemptied of their contents.

A great source of error which is, however, impossible with proper manipulation, is due to tubes being insufficiently cleaned. The result of this is, that the wall of the tubes will contain traces of substances, which react with ninhydrin if sufficiently concentrated. They may be present in such minute quantities as to be unable of themselves to produce a coloration; yet they will, when added to the analogous substances that are present in the serum, convert a negative reaction into a positive one. Therefore the utmost possible care must be exercised in the manipulation of the tubes.